1. Field of the Invention
The present invention relates to a piezoelectric ceramic material, and more particularly, to a piezoelectric ceramic material predominantly containing lead titanate.
2. Background Art
Conventionally, piezoelectric ceramics have been employed in filters used in communication apparatus and also in piezoelectric transducers such as CPU clocks and sensors. Such piezoelectric ceramics predominantly comprise a component such as PbTiO3 or PbTiO3xe2x80x94PbZrO3.
Among these ceramics, piezoelectric ceramics predominantly comprising lead titanate zirconate have a Curie temperature as low as approximately 300xc2x0 C., although they have a high electromechanical coupling coefficient. Thus, the ceramics cannot be employed at temperatures higher than the above Curie temperature. In addition, the ceramics are not suitably used in piezoelectric transducers which are employed in a high-frequency region due to their having a dielectric constant as high as approximately 1000.
Thus, a piezoelectric ceramic which is used in piezoelectric transducers particularly employed in a high-frequency region comprises lead titanate as a primary component and additives serving as secondary components such as Bi2O3, Cr2O3, MnO2 and ZnO, the lead titanate imparting a high Curie temperature and a low dielectric constant to the piezoelectric ceramic.
In addition, Japanese Patent Application Laid-Open (kokai) No. 56-2689 discloses a piezoelectric ceramic composition essentially constituted by a binary-component oxide system represented by (1xe2x88x92x)PbTiO3xe2x80x94xPb(Mexc2xdMnxc2xd)O3 (Me: Ta, Nb) in which 1.0-2.0 at. % of Pb is substituted by Sr, and at least one secondary component selected from among WO3, NiO and Fe2O3 is incorporated in an additional amount of 0.05-2.0 wt. %. The composition successfully attains lowering of sintering temperature, suppression of grain growth, improvement of temperature characteristics and time-elapsing characteristics, and improvement of mechanical quality factor.
However, when PbTiO3 containing W as a secondary component is fired, a W-containing compound or oxide is segregated in the produced sintered PbTiO3. Since such a segregated W compound or W oxide has a significant volume, small-volume chip parts formed from sintered PbTiO3 including the above segregation exhibit variation in electrical characteristics. Furthermore, mechanical strength of the sintered product decreases, thereby increasing failure of chip parts due to cracks and chipping.
In view of the foregoing, an object of the present invention is to provide a piezoelectric ceramic material which can suppress segregation of a W compound or a W oxide in W-containing sintered PbTiO3. Another object of the invention is to provide a piezoelectric transducer employing the ceramic material.
The present invention has been made so as to attain the aforementioned objects.
Accordingly, the present invention provides a piezoelectric ceramic material which comprises lead titanate as a primary component; an Mn-containing oxide or an Mn-containing compound, and a W-containing compound or a W-containing oxide as secondary components; and an Si-containing oxide or an Si-containing compound as an additional component.
The ceramic material having such a composition provides a piezoelectric ceramic having a high Curie temperature and a low dielectric constant and suppresses segregation of a W-containing oxide or compound in the ceramic. In other words, incorporation of Mn and W into lead titanate successfully lowers firing temperature and suppresses evaporation of Pb, thereby reducing variation in piezoelectric characteristics and attaining high mechanical quality factor (Qmt), while high Curie temperature and low dielectric constant, which are characteristics of lead titanate, are maintained. The present inventors have further found that an Si-containing oxide or compound suppresses segregation of a W-containing compound or oxide. Thus, incorporation of silicon oxide into lead titanate successfully suppresses segregation of a W-containing oxide or compound in piezoelectric ceramics, thereby producing piezoelectric ceramics of excellent reliability and of uniform characteristics.
The present invention also provides a piezoelectric ceramic material which comprises lead titanate as a primary component; an Mn-containing oxide or an Mn-containing compound, and a W-containing compound or a W-containing oxide as secondary components; and an Si-containing oxide or an Si-containing compound as an additional component, wherein about 15 mol % or less (0 mol % excluded) of Pb in the lead titanate is substituted by at least one element selected from among lanthanum, neodymium and cerium.
The ceramic material having such a composition provides a piezoelectric ceramic having a high Curie temperature and a low dielectric constant; lowers sintering temperature; and suppresses evaporation of Pb, thereby reducing variation in piezoelectric characteristics and attaining high mechanical quality factor (Qmt). The ceramic material also suppresses segregation of a W-containing oxide or compound in a produced ceramic. In addition, the temperature characteristics of piezoelectric ceramics can be enhanced as compared with the case of piezoelectric ceramics produced from a non-substituted similar ceramic material.
Preferably, the Mn-containing oxide or compound is incorporated in an amount of about 0.1-5.0 wt. % as reduced to MnO, and the W-containing oxide or compound is incorporated in an amount of about 0.1-5.0 wt. % as reduced to WO3, the weight percentages being on the basis of the primary component. According to this mode, lowering sintering temperature and enhancement of mechanical quality factor can be attained to a greater degree.
Preferably, the Si-containing oxide or compound is added in an amount of about 2.0 wt. % or less (0 wt. % excluded) as reduced to SiO2, with respect to the sum of the above primary component and the above secondary components. According to this mode, segregation of a W-containing oxide or compound can be effectively suppressed without affecting other characteristics, and sintering temperature can be lowered.
The present invention also provides a piezoelectric ceramic material which comprises a primary component containing PbTiO3 and Pb(MnyW(1xe2x88x92y))O3 and additionally SiO2 or PbSiO3. The ceramic material having such a composition provides a piezoelectric ceramic having a high Curie temperature and a low dielectric constant, and suppresses segregation of a W-containing oxide or compound in a produced ceramic.
The present invention also provides a piezoelectric ceramic material which comprises a primary component containing (Pb(1xe2x88x923x/2)Mx)TiO3 and Pb(MnyW(1xe2x88x92y))O3 (M represents at least one species selected from among La, Nd, and Ce; 0 less than xxe2x89xa60.1; and 0 less than y less than 1) and additionally SiO2 or PbSiO3. The ceramic material having such a composition provides a piezoelectric ceramic having a high Curie temperature and a low dielectric constant; lowers sintering temperature; and suppresses evaporation of Pb, thereby reducing variation in piezoelectric characteristics and attaining high mechanical quality factor (Qmt). The ceramic material also suppresses segregation of a W-containing oxide or compound in a produced ceramic. In addition, temperature characteristics of piezoelectric ceramics can be enhanced as compared with the case of piezoelectric ceramics produced from a non-substituted similar ceramic material.
Preferably, in the above piezoelectric ceramic material, the primary component comprises xe2x80x9caxe2x80x9d mol of (Pb(1xe2x88x923x/2)Mx)TiO3 and xe2x80x9c(1xe2x88x92a)xe2x80x9d mol of Pb(MnyW(1xe2x88x92y))O3 (0.9xe2x89xa6axe2x89xa60.99; xc2xdxe2x89xa6yxe2x89xa6⅔). According to this mode, lowering sintering temperature and enhancement of mechanical quality factor can be attained to a greater degree.
Preferably, the above piezoelectric ceramic material comprises 100 wt. % of the aforementioned primary component and about 2.0 wt. % or less (0 wt. % excluded) of additional SiO2 or PbSiO3 as reduced to SiO2. According to this mode, segregation of a W-containing oxide or compound can be effectively suppressed without affecting other characteristics, and sintering temperature can be lowered.
The present invention also provides a monolithic piezoelectric transducer which comprises a plate formed of the above-recited piezoelectric ceramic material, a vibration electrode and an external electrode, wherein the vibration electrode is disposed on the piezoelectric ceramic plate to form a laminated body, and the external electrode is electrically connected to the vibration electrode included in the laminated body.
The monolithic piezoelectric transducer has excellent heat resistance and is suitable for employment in a high-frequency region.